Electron Beam Topoff Injection in JLEIC Fanglei Lin

Electron Beam Top-off Injection in JLEIC Fanglei Lin, Jiquan Guo, Yuhong Zhang, Mark Wiseman, Marcy Stutzman JLEIC R&D Meeting, Nov. 14, 2019

Radiative Polarization • Nov 14, 2019 JLEIC R&D Meeting 2

Electron Polarization Configuration in Figure-8 JLEIC • • Two depolarization sources: i) Sokolov-Ternov effect, ii) spin diffusion effect Equilibrium polarization due to these two depolarization sources is zero Two co-existing polarization states experience the same decay rate Top-off injection (i. e. continuous injection) of highly polarized electron beam can maintain high final equilibrium polarization Nov 14, 2019 JLEIC R&D Meeting 3

Top-off Injection Bunch Train Mid-cycle 1, inject the 1 st of every 7 buckets in the ring Polarization Down Polarization Up 68. 05 MHz bunch train, 3. 9 µs, Ipulse = 2 m. A 14. 69 ns, 22 CEBAF or 7 ring buckets 2. 1 ns, 476. 3 MHz (1 ring bucket ) 1 ps, 30 p. C bunch …… …… τw = 0. 06 -8 s Iinj = 1 - 127 n. A τw = 0. 06 -8 s Determined by 85% polarization Nov 14, 2019 JLEIC R&D Meeting 4

Polarization vs. Averaged Injection Current P = 85% • Assume 90% polarization from CEBAF. • With an average injected current up to 127 n. A, an equilibrium polarization of 85% can be achieved in the whole energy range. Nov 14, 2019 JLEIC R&D Meeting 5

Polarization vs. Beam Lifetime 6 minutes 32 minutes 1 minute 24 seconds • Assume 90% polarization from CEBAF. • The shortest beam lifetime is 24 seconds at 12 Ge. V electron beam energy in order to keep equilibrium polarization of 85%. Nov 14, 2019 JLEIC R&D Meeting 6

Top-off Parameters (assuming no damping wiggler) Item Unit Parameter Ge. V 3 5 7 10 12 A 3 3. 6 3 0. 82 0. 39 % 90 % 85 Goals h 116 9. 0 1. 7 0. 28 0. 11 Calculated results n. A 0. 95 14. 7 66 107 127 h 6. 8 0. 53 0. 098 0. 017 0. 0067 Required in order to reach 85% polarization s 8. 2 0. 53 0. 12 0. 07 0. 06 Required in order to keep required average injection current 1070 1524 Resulted 1/17 W Nov 14, 2019 2. 85 73. 5 JLEIC R&D Meeting 462 7

Average Injection Current and Beam Lifetime vs. Beam Energy Polarization is 85% in the electron energy range of 3 to 12 Ge. V 6. 8 h 127 n. A 107 n. A 66 n. A 32 min 0. 95 n. A Nov 14, 2019 6 min 1 min 24 s 14. 7 n. A JLEIC R&D Meeting 8

Shorten Beam Lifetime Schemes to shorten beam lifetime Local control Scrape the beam of up to 1. 5 k. W beam power Control of potential issues • One or two local scrape(s) with local pump(s) to remove the heat (see slide 10) • Active radiation shield may be needed - for example, 4 -8 inches of lead to shield ~ 2 k. W beam power in dumplets in CEBAF Global control of up to 1. 5 k. W beam power ( < 1 W/m) Reduce the dynamic aperture Use fast feedback system to destabilize a specific bucket before re-injection (Andrew Hutton) • Pumps in the whole ring to remove the heat • Active radiation shield may not be needed (? ) Slide 11 for more ideas/comments in detail Nov 14, 2019 JLEIC R&D Meeting 9

Local Scraping and Pumping Sp in r ota t or Arc Tune trombone, straight FODO, RF or in Sp at rot Arc 70. 94 o Future IP in Sp tor a t ro IP Sp in rot a tor Forward e- detection, polarimetry Scrape the beam locally, after the downstream crab cavities and before the arc (spin rotator). Local pump following the scraping can remove most of the heat immediately. Pumps for arc dipoles can remove the rest of heat. Nov 14, 2019 JLEIC R&D Meeting 10

Ideas and Comments Single bunch injection => Source needs generate a few of n. C polarized electron bunch; CEBAF needs tolerate electron beams with a large energy spread caused by the cavity voltage droop. • From Marcy Stutzman: －Would it make any sense to add a potential RF kicker to kick the unwanted beam down a beam tube or into a pumping and absorbing area? The deflection needed would be small. • From Mark Wiseman: －I think it would be best if we can use a kicker, etc. to put the beam in a spot we can design to and control. －If not, 1. 5 k. W of diffuse loss in the arcs would probably not be a problem for heating as there is plenty of cooling in the vacuum chambers. I think there is probably adequate vacuum pumping as well. We already have 10 k. W/m of SR radiation to control. I think the SR shields on the RF bellows would help act as a shield for this as well. Of course any concentrated power could get us in trouble. If the loss is in the straights, then we should review the designs to make sure. I just don't know the straight designs well enough to say off hand. It really depends on how diffuse the beam loss is. In either case we should consult with radiation safety on this. It is certainly less than ideal. －Certainly we don't want any losses in the cryogenic sections, IP magnets and IP chambers, Crab cavities, etc. Nov 14, 2019 JLEIC R&D Meeting 11

Other Studies are Needed • Injection kicker with long pulse (us) vs. short pulse • Measure the beam current • Polarization lifetime with wigglers Nov 14, 2019 JLEIC R&D Meeting 12

Back Up Nov 14, 2019 JLEIC R&D Meeting 13

Electron Polarization Strategies • Highly vertically polarized electron beam from CEBAF －High equilibrium polarization maintained by continuous injection －Two oppositely polarized bunch trains • Polarization vertical in the arc to avoid spin diffusion and longitudinal at collision points －Universal spin rotator with fixed orbit from 3 to 12 Ge. V • Energy independent spin tune • Spin matching considered • Compton polarimeter Nov 14, 2019 JLEIC R&D Meeting 14